Most of the liquid producing oil wells which require artifical lift employ a motor drive pumpjack which reciprocates a string of sucker rods. The rods extend downhole several thousand feet to a production pump located in proximity of a production zone. As the motor drives the pumpjack apparatus, the sucker rod string causes the downhole pump to lift fluid to the surface of the earth. As the pumping action continues, the reservoir fluid level contained within the borehole annulus will usually be lowered until it recedes below the pump inlet, whereupon further pumping action induces an undesirable condition called "fluid pounding". Fluid pounding is caused by the pump piston accelerating in a downward direction through a gaseous phase, whereupon the piston encounters the liquid phase with sufficient force to produce a shock wave having a magnitude dependent upon the quantity of gaseous products ingested into the pump barrel. The shock wave causes damage to the entire production apparatus, and has been the subject of many different novel pump-off control means, as evidenced by the following patents, to which reference is made for further background of the invention.
______________________________________ Mills 3,851,995 Schmidly Jr. 3,509,824 Elliot et al 3,073,244 Hubby 3,413,535 Hubby 3,440,512 Borger et al 3,138,750 Aselman Jr. Re 27,393 Jaeger 3,363,573 ______________________________________
Ideally, a pump-off control unit would directly measure the reservoir fluid level respective to the pump inlet, and control the pump in such a manner that the liquid level is always maintained above but in close proximity to the pump inlet, thereby realizing maximum production from the well while completely avoiding a pump-off condition. Since it is not feasible to measure a liquid level contained thousands of feet downhole in a borehole, it is therefore desirable to measure some variable associated with the production apparatus which changes in response to a change in reservoir fluid level. By utilizing this hypothetical variable for controlling the operation of the pumpjack apparatus, a severe pump-off condition can be avoided, while at the same time, a maximum production rate will be realized from the borehole.